1. Field of the Invention
The present invention relates to an image pickup device capable of automatic exposure control (AE) and manual exposure control.
2. Related Background Art
Recent remarkable advancements in video tape recorders (VTR) and other imaging equipment have reached a stage that image taking equipment such as so-called cam-corders are equipped with automatic exposure control device as a standard feature, and some are even equipped with manual exposure control as an additional feature.
In the following is the conventional exposure control system in such cam-corders, is explained with reference to FIG. 1.
At first, in the automatic exposure control state, the light coming from an object is subjected to an adjustment in quantity by a diaphragm 2 constituting an exposure control mechanism, and is focused on a light receiving face of an image pickup element 3. The object image focused on said image pickup element 3 is photoelectrically converted, and an obtained image signal is supplied to a camera signal processing circuit 4, which effects image processing such as gamma conversion to obtain a color signal C and a luminance signal Y.gamma. as the image signals. These image signals are guided through a camera encoder 5 for example of NTSC format and released to the exterior in the form of a composite image signal.
Also a luminance signal Y released from said camera signal processing circuit 4 is supplied to a detection circuit 6 for generating a control signal for controlling the diaphragm 2 so as to obtain an appropriate exposure according to the luminance of the object. Said detection circuit 6 effects, for example, integration detection of said luminance signal, and compares the integrated signal in an operational amplifier 7 with a reference voltage from a voltage generator 8, and the difference is amplified and supplied to a driver 10. A control signal from said driver 10 controls the diaphragm 2 through an actuator 11 therefor. An output signal of an encoder 12 for detecting the state of the diaphragm 2 is differentiated by a differentiating circuit 13, and is supplied as a negative feedback signal to the driver 10 for controlling this circuit.
In the following, manual exposure control operations are explained. When an exposure operation selector switch 9 is shifted from automatic to manual, the output signal of said encoder 12 for detecting the state of the diaphragm 2 is compared, in an operational amplifier 14 with a reference voltage of a voltage generator 15 for manually setting the diaphragm aperture, and the obtained difference is supplied from said operational amplifier 14 to the driver 10 of the above-explained diaphragm driving circuit. The control is conducted in a similar manner as in the automatic exposure control explained above, so as that the output voltage of the encoder 12 becomes equal to the reference voltage of the voltage generator 15, whereby the output of image pickup element 3 becomes constant.
In such an image pickup device capable of automatic and manual exposure control, in which the operation of the actuator for controlling the diaphragm is determined by a feedback circuit involving an encoder, since somewhat strong feedback is adopted for reaching a stabilized state within a certain operation time, in order to achieve a smooth diaphragm control operation in the automatic exposure control, the manual exposure control is conducted with a safe feedback rate as in the automatic exposure control. For this reason, in the automatic exposure control state, the diaphragm requires a long driving time, thus being unable to regulate the aperture with satisfactory response, and incapable of smooth diaphragm regulating operation.
In the field of exposure control, the present applicant already made certain proposals for example in the U.S. patent applications Ser. No. 484,539 filed Feb. 23, 1990 and Ser. No. 393,644 filed Aug. 14, 1989.